Very Large Antenna Systems
Due to recent technology and standardization development, introducing large antenna arrays at cellular base stations and other wireless access points has become a viable option to boost the capacity and user data rates. A base station (BS) or an access point (AP) equipped with excessive number of antennas, can simultaneously schedule multiple user equipment (UEs) at the same time/frequency band with simple linear processing such as maximum-ratio transmission (MRT) or zero-forcing (ZF) in the downlink (DL) and maximum-ratio combining (MRC) or ZF in the uplink (UL). This is often referred to as very-large (or full dimension, FD) multiple-input multiple-output (VL-MIMO) or massive MIMO in the literature. The gains with VL-MIMO are achieved without consuming any additional spectrum. Additionally, the radiated energy efficiency with VL-MIMO can be substantially improved. Recognizing the technology potential, the 3GPP has defined a work item on VL-MIMO.
A key usage of VL-MIMO technology is (extreme) narrow beam forming for DL transmissions, that enables a BS to focus the transmitted energy to the desired UE rather and thereby boosting the coverage and user data rates for DL transmissions. For VL-MIMO systems it is not trivial how to acquire channel state information (CSI) in a scalable fashion, which is essential to gain the performance potentials of excessive amount of transmit antennas. Traditional schemes where each UE continuously measures on the pilot (reference) symbols transmitted by the BS during downlink transmission phase to estimate the downlink channel gain and feeds it back to the BS via a reverse link, would not work in VL-MIMO systems. This is so because the number of required pilots in the downlink is proportional to the number of BS antennas, and hence feedback based schemes are not scalable. Existing solutions to this problem operate in the time-division duplex (TDD) mode and rely on the channel reciprocity between the uplink and the downlink. Note that in order to guarantee UL/DL channel reciprocity some hardware calibrations might be needed in practice. Then, the channel estimate of the uplink direction at the transmitter can directly be utilized in the downlink.
Wireless Channel Sounding
In existing systems, wireless channel sounding refers to the mechanism that enables a wireless device access point or BS to obtain wideband uplink channel state information in parts of the spectrum, in which currently no wireless data transmission is taking place. Specifically, in cellular systems, a BS has at least two usages of wideband channel sounding:                To acquire UL channel state information in frequency and time resources in which a UE is currently not scheduled (even though the UE may currently use other parts of the spectrum);        To acquire UL channel state information of UEs that are currently not transmitting uplink data.        
In Long-Term Evolution (LTE), channel sounding is done via the so-called sounding reference signals (SRS) transmitted by each UE. The exact structure of the SRS can be found in [1] but in short, the SRS is transmitted at the last Orthogonal Frequency-Division Multiplex (OFDM) symbol during UL as frequency reference signals inserted into every second subcarrier creating a comb-like spectrum. The minimum SRS bandwidth is 4 resource blocks (720 KHz). There exist two types of SRS transmissions:                1) Periodic SRS: In which, the UE is transmitted SRS with a given configured periodicity (can be as often as once every 2 ms to once every 160 ms).        2) Aperiodic SRS: In which, the UE transmits SRS upon receiving explicit command through Physical Downlink Control Channel (PDCCH) signaling.Demodulation Reference Signal        
In existing systems, demodulation reference signals (DMRS) are used to enable the coherent demodulation of the transmitted data. More precisely, the DMRS is inserted in-band with the data so that it goes through the same processing chain as does the data, which ultimately enables the coherent demodulation of the data. Herein, the data includes any type of information to be communicated including DL payload data (transmitted for example in LTE physical downlink shared channel—PDSCH), UL payload data (transmitted for example in LTE physical uplink shared channel—PUSCH), DL control signaling (transmitted in LTE physical downlink control channel—PDCCH) and UL control signaling (transmitted in LTE physical uplink control channel—PUCCH).
Note: For the ongoing 5G discussions, the nomenclature and definition of uplink pilot/reference signals are not yet decided. However, since both UL SRS and UL DMRS have very similar structures, it is predicted that the UL SRS and UL SRS are merged into a single UL reference signal. Additionally, since VL-MIMO is a key component of 5G and since in VL-MIMO systems DL channel knowledge is obtained via uplink channel sounding, sufficient UL SRS-type pilot signals are necessary in 5G. We call these pilots Reciprocity Reference Signals (RRS) throughout the rest of this disclosure. Observe that, to accommodate more UEs in 5G, there might be a need to introduce further enhancements to the current SRS structure in order to be able to use SRS as RRS. For instance, more comb structures might be introduced, or we might allow RRS transmission to take place in several OFDM symbols (as opposed to SRS transmission which occupied the last OFDM symbol).
Hybrid Automatic Repeat Request
To facilitate reliable data transmission, there is a need to retransmit the erroneously received packets. This is done with the help of Automatic Repeat Request (ARQ) or Hybrid-Automatic Repeat Request (HARQ) mechanisms. The basic idea is that the receiver, upon reception of a new error-free packet, sends a positive acknowledgement (ACK) to the transmitter via a reverse link to inform the error-free reception of the packet and correspondingly a negative acknowledgement (NACK) to inform that the packet is in error in case that the packet is received erroneously. Then the transmitter retransmits the packets for which it has received NACK.
In LTE, the ACK/NACK signals corresponding to the UL payload transmissions are transmitted in the so-called Physical HARQ Indicator Channel (PHICH). In the UL, the ACK/NACK messages are transmitted either on the Physical Uplink Control Channel (PUCCH) or together with payload data on Physical Uplink Share Channel (PUSCH) depending on whether the UE has an active UL session with the BS, see [2] for more information.
Scheduling Request (SR)
A Long-Term Evolution (LTE) UE is configured by the network with one particular code and time/frequency resource which together constitute a Scheduling Request (SR). The UE will then request scheduling of uplink resources (PUSCH) by sending a PUCCH message on its given SR resources. The network, upon receiving the SR, will respond via a scheduling grant which specifies the UE identity and the UL resources that are assigned to the UE. For further details, see Section 5.4.4 of [2].
If the network node knows the downlink channel towards the UE, then it can efficiently use this knowledge to more effectively transmit the response. This gain is especially high in VL-MIMO systems where the BS is equipped with 100-400 antennas. As described above, for VL-MIMO systems, it is not trivial how to acquire channel state information (CSI) in a scalable fashion. Therefore there is a need, especially in 5G, for a more efficient procedure for UL scheduling request.